The present invention relates to an optical system especially for use in binoculars. More particularly, the invention relates to an automatic focussing system for use with binoculars in which a pair of individual light entry windows for distance metering are provided near right and left objectives for introducing light rays via movable and fixed reflecting members into a focus detecting element module.
In most prior art binoculars constructions, the focus adjustment was carried out by manually turning an adjustment ring for the objectives to thereby make coincide a front focus of the oculars with a position of the focussing plane of the objectives.
However, it is time-consuming and somewhat inaccurate for the focussing operation to be carried out by moving the oculars in this manner. Therefore, the applicants previously proposed binoculars with an automatic focussing system in U.S. Patent Application Ser. No. 74,354 filed on Sept. 10, 1979 which is cited here not as prior art but is mentioned merely as disclosing a related device to aid in an explanation of the present invention. Binoculars as disclosed therein are shown in FIG. 5. In these binoculars, in the oculars, a fixed partially reflecting member b and a movable partially reflecting member b' made of semitransparent material are disposed following right and left objectives a and a'. Light rays passing through the partially reflecting member b and b' are introduced to oculars d and d' through prisms c and c' and simultaneously therewith the light rays reflected by the reflecting members b and b' are introduced to a focus detecting element module e. The movable reflecting member b' is so moved that a dual image formed by the reflecting member b and b' coincides on the focus detecting element module e while at the same time the oculars d and d' are moved, for example, by a step-up motors, cooperating with the movement of the movable reflecting member b' whereby automatic focussing is performed.
However, the above-described binoculars are somewhat disadvantageous in that, since a pair of objectives per se are used as light entry windows and the fixed and movable reflecting members are made of semitransparent reflex mirrors, the total light entering through the objectives a and a' is divided between the objective lenses and the focus detecting element module as a result of which the image through the objectives is reduced in intensity and in some cases an insufficient amount of light is supplied to the focus detecting element module. This leads to difficulties in obtaining good focussing accuracy if the light level is low.